Application Ser. Nos. 985,411 and 985,406 filed Dec. 4, 1992 assigned to the same assignee as the instant invention relate to a power controller which serves as the basis for the present invention.
This invention relates generally to a solid state circuit controller and more specifically to a solid state circuit controller for switching power to a load and for protecting the switch from damage due to power dissipation.
In the above referenced copending applications, a solid state circuit controller is described and claimed which has utility in many applications, such as, by way of example, in aircraft power distribution systems.
The controller described in those applications include overload current wire protection referred to as the I.sup.2 RC section, overload current limit referred to as the MOSFET Drive and Current Limiting section, a temperature rise MOSFET Protection section and various other support apparatus to facilitate a power controller function. In the overload current wire protection section a voltage is produced which is proportional to the current flow in a protected circuit. The voltage is converted to a proportional current, squared and then integrated as a function of time to provide a signal proportional to the temperature rise of the circuit wire. A trip signal is produced when the temperature rise signal reaches a maximum as prescribed by an appropriate time--current relationship. The section comprises a linear circuit which measures the current flow through the wire, calculates its heating impact on the wire and provides a signal to trip the controller when the wire temperature has risen to its maximum safe level as defined by a selected curve set such as one derived from the MS3320 trip time vs load current curves. The circuit reads the current in the wire as a voltage signal across a shunt resistor serially connected to the wire, a signal is scaled by an amplifier stage, squared with the resultant applied to an RC network. The output voltage from the RC network is monitored by a comparator which has a threshold set by the referenced voltage according to an energy equation. When the output voltage of the RC network reaches the threshold level the comparator output transitions to an opposite state signaling the Logic section of a trip condition in order to interrupt current flow within the wire by turning off the MOSFETs.
The MOSFET Drive and Current Limiting section turns the MOSFETs on and off by supplying and removing, respectively, MOSFET gate charge and limits current flow through the MOSFETs to a predetermined level by decreasing and increasing the MOSFETs on resistance through gate charge addition and removal, respectively.
The controller provides a current signal proportional to the voltage drop across the MOSFET switch when the controller is current limiting. Because MOSFET current is constant at this point the current signal provided is proportional to the power dissipation within the MOSFET switch. The current signal is applied to an RC network to integrate over time arriving at the stored energy and temperature rise of the MOSFET junction. The output voltage of the RC network is monitored by a comparator which has a threshold set to represent the maximum allowable temperature of the MOSFET switch junction. When the output voltage of the RC network reaches the threshold level the comparator output transitions to an opposite state signaling the Logic section of a trip condition in order to interrupt current flow by turning off the MOSFETs.
It is an object of the present invention to provide in-situ, external programmability of a solid state controller in order to allow adjustment of certain features including current overload protection, MOSFET thermal protection and current limit as desired for different applications.
Various other objects and advantages will appear from the following description of an embodiment of the invention and the novel features will be particularly pointed out hereinafter in connection with the accompanying drawings and the appended claims.